Annular combustion chamber with two offset heads

ABSTRACT

An offset annular combustion chamber for an airplane engine gas turbine, the chamber comprising a pilot head having a plurality of nozzle systems distributed on a pilot head chamber end wall interconnecting an inner longitudinally-extending side wall of the chamber to a pilot head outer longitudinally-extending side wall, and a take-off head that is radially and axially offset from the pilot head and comprising a plurality of nozzle systems distributed on a take-off head chamber end wall interconnecting the pilot head outer longitudinally-extending side wall and a take-off head outer longitudinally-extending side wall, the pilot head having at least N substantially identical nozzle systems with overall permeability PA adapted to lighting and to speeds close to idling, and the take-off head having at least 2N substantially identical nozzle systems of overall permeability PB, where PB is greater than or equal to PA, the permeability PA lying in the range 10% to 40% of the total air flow rate penetrating into the chamber and the permeability PB lying in the range 30% to 70% of the total air flow rate penetrating into the chamber.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the field of annular combustionchambers with two offset heads for an airplane engine gas turbine. Theinvention relates more particularly to the arrangement of fuel nozzlesystems fitted to such combustion chambers.

[0002] A gas turbine combustion chamber is formed in conventional mannerby inner and outer annular side walls extending longitudinally andunited by a chamber end wall. Combustion reactions take place within thechamber, and the chamber is configured so that the flow of air that itreceives is shared between at least three fractions: combustion air;dilution air; and air needed for cooling the chamber walls, which airdoes not participate directly in combustion phenomena. The chamber thuscomprises a primary or combustion zone and a secondary zone known as a“dilution” zone which is situated downstream from the preceding zone.

[0003] Fuel is fed to the combustion chamber via fuel nozzles placed inholes that pass through the end wall of the chamber. Air for thecombustion zone is introduced into the chamber, partly through its endwall and possibly via the nozzles, and partly via through orificespierced in the longitudinally-extending side walls. Dilution air isgenerally introduced further downstream in the combustion chamber viaone or more rows of holes that are likewise distributed in the sidewalls of the chamber.

[0004] By their very design, the combustion chambers that are presentlyin use make it difficult to minimize polluting emissions that come fromcombustion, in particular emissions of nitrogen, of carbon monoxide, andof unburnt hydrocarbons.

[0005] In order to solve this problem, it is known to use combustionchambers having two heads that are offset, i.e. chambers in which thefuel nozzles are shared between a so-called “pilot” head and a so-called“take-off” head that is spaced apart from the pilot head both radiallyand axially, the take-off head being situated downstream from the pilothead in the direction of gas flow in the chamber.

[0006] Conventionally, the “pilot” nozzles are used for lighting andwhen the engine is idling, while the “takeoff” nozzles are used during“full throttle” (FT) stages, in particular during take-off and whilecruising. In general, the pilot nozzles are fed with fuel continuously,while the take-off nozzles are fed only above some determined minimumspeed.

[0007] Thus, for example, document FR 2 727 193 discloses an annularcombustion chamber in which the nozzles are distributed over a pilothead and a take-off head. The pilot head is fitted with n nozzles ofpermeability P1 adapted to idling the speed. The take-off head is alsofitted with n nozzles of permeability P1 enabling the take-off head tobe lighted at low speed, plus n take-off nozzles of permeability P2>P1adapted to full load conditions (where the term “permeability”concerning n nozzles relates to the total flow of air passing throughall n nozzles).

[0008] The permeability P1 of the n pilot nozzles lies in the range 10%to 12% of the total air flow that penetrates into the combustionchamber. Given the head loss due to the air going past the pilot head inorder to reach the take-off head, the same permeability P1 for the nfirst take-off nozzles corresponds to about 8% to 10% of the totalincoming air flow. In contrast, the permeability P2 of the n secondtake-off nozzles is 26% to 35% of said total air flow.

[0009] Such a disposition makes it easier to switch between idlingconditions, in which only n pilot nozzles are fed, and a sector burning(SB) situation in which only the n take-off nozzles of permeability P1are lighted amongst the take-off nozzles.

[0010] However, because of the large difference between the permeabilityP2 and the permeability P1 of the take-off nozzles, the subsequentswitch from sector burning (SB) to full throttle (FT) is more difficult.It can be achieved only with feed that is relatively rich, and thus withthe engine turning at high speed (it is stated in document FR 2 727 193that the n take-off nozzles of permeability P2 are lighted once the highpressure compressor has reached 70% of its nominal full throttle speedof rotation).

[0011] Unfortunately, prolonged operation under sector burningconditions presents drawbacks: temperature distribution over the highpressure turbine blades is not optimum, and alternating lighted nozzlesand nozzles that are out in the take-off head encourages chemicalreactions to “freeze”, thus affecting combustion efficiency andencouraging undesirable emissions of particles and unburnt fuel.

OBJECT AND SUMMARY OF THE INVENTION

[0012] The present invention seeks to mitigate those drawbacks byproposing an annular combustion chamber with two offset heads thatprovides an operating range that is significantly extended compared withconventional technologies using one or two heads, while neverthelessgiving control over temperature profiles and reducing pollutingemissions.

[0013] To this end, the invention provides an offset annular combustionchamber for an airplane engine gas turbine, the chamber comprising apilot head having a plurality of nozzle systems distributed on a pilothead chamber end wall interconnecting an inner longitudinally-extendingside wall of the chamber to a pilot head outer longitudinally-extendingside wall, and a take-off head that is radially and axially offset fromthe pilot head and comprising a plurality of nozzle systems distributedon a take-off head chamber end wall interconnecting the pilot head outerlongitudinally-extending side wall and a take-off head outerlongitudinally-extending side wall, the pilot head having at least Nsubstantially identical nozzle systems with overall permeability PAadapted to lighting and to speeds close to idling, and the take-off headhaving at least 2N substantially identical nozzle systems of overallpermeability PB, where PB is greater than or equal to PA, wherein thepermeability PA lies in the range 10% to 40% of the total air flow ratepenetrating into the chamber and the permeability PB lies in the range30% to 70% of the total air flow rate penetrating into the chamber.

[0014] Using 2N nozzle systems having the same individual permeabilityfor the take-off head makes it possible to ensure that changeover takesplace under good conditions both from idling to SB and from SB to FT,where it is possible to perform the SB to FT changeover while runningslowly, even close to idling.

[0015] In advantageous dispositions, the permeability PA lies in therange 17% to 21% of the total air flow penetrating into the combustionchamber, and the permeability PB lies in the range 36% to 45% of thesame air flow.

[0016] The outer longitudinally-extending side wall of the take-off headand possibly also the outer longitudinally extending side wall of thepilot head, together with the inner side wall advantageously have rowsof dilution orifices. The air flow rate penetrating via said dilutionorifices lies in the range 4% to 10% and preferably in the range 6% to8% of the total air flow rate entering the chamber for the outerorifices in the outer longitudinally-extending side wall(s), and lies inthe range 2% to 8% and preferably in the range 4% to 6% for the innerorifices formed in the inner side wall.

[0017] The axes of the nozzle systems of the pilot and take-off headsare advantageously directed towards a common annular zone for exhaustingthe gases that come from combustion.

[0018] The nozzle systems of the pilot and take-off heads are installedon end walls of the chamber which may be perpendicular to the axis ofthe engine or which may be conical in shape.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other characteristics and advantages of the present inventionappear from the following description given with reference to theaccompanying drawings which show an embodiment that is not limiting inany way. In the figures:

[0020]FIG. 1 is a highly diagrammatic axial half-section view of acombustion chamber constituting an embodiment of the invention;

[0021]FIG. 2 is a highly diagrammatic fragmentary view showing oneexample of how the nozzle systems can be distributed on the end walls ofthe pilot and take-off heads; and

[0022]FIG. 3 is a section view showing a particular embodiment of anozzle system in accordance with the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0023] Reference is made initially to FIG. 1 which is a diagrammaticaxial half-section showing a combustion chamber 1 constituting anembodiment of the invention. The axis referenced X-X corresponds to theaxis of the engine fitted with such a combustion chamber.

[0024] The combustion chamber 1 is of the annular type with a pilot head12, and with a take-off head 14 that is offset from the pilot head bothradially and axially, the take-off head being situated downstream fromthe pilot head in the direction of gas flow in the chamber. The chamberis formed in particular by an outer longitudinally-extending side wall 2for the take-off head, an inner longitudinally-extending side wall 4,and an outer longitudinally-extending side wall 6 for the pilot head.The transverse end wall 8 of the chamber for the pilot head unites thepilot outer side wall 6 and the inner side wall 4, while the take-offhead outer side wall 2 and the pilot head outer side wall 6 are unitedby an end wall 10 of the take-off head, likewise extending transversely.

[0025] As shown in FIG. 2, fuel nozzle systems 16, 18 are placed inholes 16 a and 18 a passing through the respective end walls 8 and 10 ofthe pilot and take-off heads. More precisely, the pilot head 12 has Nfuel nozzle systems 16 that are substantially identical and that areregularly distributed around the axis X-X, while the take-off head 14has 2N nozzle systems 18 that are substantially identical and that arelikewise regularly distributed around the axis X-X.

[0026] Thus, in an angular sector of the chamber corresponding to 2π/N,there are to be found two nozzle systems 18 of the take-off head foreach nozzle system 16 of the pilot head. The nozzle systems areadvantageously disposed substantially in a staggered configuration. Theterm “staggered” is used to mean that in an angular sector of 2π/N, theangular position of the pilot head nozzle system 16 is situated atsubstantially equal distances from the angular positions of the twonozzle systems 18 of the take-off head.

[0027] The pilot and take-off heads 12 and 14 may be fitted with anyconventional type of nozzle system serving either to spray fuel inmechanical, aerodynamic, or premixed manner, or else to vaporize fuel. Aparticular embodiment of a nozzle system is described below withreference to FIG. 3.

[0028] The N nozzle systems of the pilot head 12 have overallpermeability PA while the 2N nozzle systems of the take-off head 14 haveoverall permeability PB, where PB is greater than or equal to PA.Advantageously, 2PA≦PB≦3PA, and preferably 2.5PA≦PB≦3PA.

[0029] The term “overall” permeability, PA or PB, is used to designatethe air flow rates passing respectively through all N nozzle systems ofthe pilot head and through all 2N nozzle systems of the take-off head,i.e. PA=Npa, where pa is the individual permeability of each nozzlesystem of the pilot head, and PB=2Npb where pb is the permeability ofeach nozzle system of the take-off head. These permeabilities areexpressed herein as percentages of the total air flow rate penetratinginto the combustion chamber.

[0030] In advantageous dispositions, the permeability PA lies in therange 10% to 40% and preferably in the range 17% to 21% of the total airflow rate penetrating into the combustion chamber, while thepermeability PB lies in the range 30% to 70%, and preferably in therange 36% to 45% of the same air flow rate.

[0031] In conventional manner, the longitudinally-extending side wall 2of the take-off head and the inner side wall 4 may each be pierced by atleast one respective row of dilution orifices of diameter that isadjusted as a function of the required performance. These dilutionorifices enable the combustion chamber to be fed with the air requiredfor diluting the combustion gases.

[0032] The dilution orifices are preferably distributed as follows:

[0033] the outer longitudinally-extending side wall 2 of the take-offhead has at least one row of 2N outer dilution orifices 20, e.g.identical orifices, opening out into the combustion chamber 1substantially perpendicularly to the side wall, downstream from thetake-off head, and angularly distributed in regular manner about theaxis X-X; and

[0034] the inner side wall 4 has at least one row of 2N inner dilutionorifices 22 opening out into the combustion chamber substantiallyperpendicularly to the side wall and angularly distributed in regularmanner about the axis X-X.

[0035] The 2N dilution holes 22 may be distributed as N first identicalholes occupying the same angular positions as every other nozzle system18, and N second identical holes occupying the same angular positions asthe remaining nozzle systems 18, with the first dilution holes beingidentical or not identical to the second dilution holes.

[0036] The outer longitudinally-extending side wall 6 of the pilot headmay also be provided with outer dilution orifices 20′, and additionalinner dilution orifices 22′ could then be provided through the innerwall 4, substantially at the same distance along the combustion chamber,in register with the orifices 20′.

[0037] As an indication, the fraction of the air flow rate penetratinginto the combustion chamber via the dilution orifices 20 situated in theouter longitudinally-extending wall 2 of the take-off head (and togetherwith the orifices 20′, if any, situated in the outerlongitudinally-extending side wall 6) may lie in the range 4% to 10% andpreferably in the range 6% to 8% of the total flow rate, while the flowrate through the orifices situated in the inner side wall 4 may lie inthe range 2% to 8%, and preferably in the range 4% to 6% of the sameflow rate.

[0038] The remaining air flow rate is for cooling thelongitudinally-extending side walls and the end walls of the chamber.For this purpose, the longitudinally-extending side walls 2, 4, and 6 ofthe chamber are conventionally cooled by these walls being multiplyperforated or by fitting these walls with tile devices or with filmdevices.

[0039] In addition, if the chamber is lighted by means of a conventionaldevice (not shown) of the semiconductor or air igniter plug type, it canbe placed on the axis of one of the nozzle systems 16 of the pilot head12, for example.

[0040] Advantageously, the chamber end walls 8, 10 and the nozzlesystems 16, 18 which pass through them are disposed in such a mannerthat the axes of the nozzle systems point towards a common annular zonefor exhausting the gases generated by combustion. For this purpose, FIG.1 shows two possible dispositions for the chamber end walls and theirrespective nozzle systems: in continuous lines the end walls 8 and 10are substantially perpendicular to the axis X-X of the engine, while indashed lines they are substantially frustoconical in shape. In the firstcase, the axes Y, Z of the nozzle systems 16, 18 can be inclinedrelative to the normal to the chamber end walls 8 and 10, whereas in thesecond case they can be perpendicular to the chamber end walls.

[0041] These dispositions serve to reduce as far as possible the totalvolume of the combustion chamber and to improve performance in terms oftemperature, combustion efficiency, and reducing polluting emissions.The convergence of the nozzle system axes serves to increase mixingspeed and the rate at which fuel burns in the chamber, and consequentlyencourages complete combustion of the fuel in a small volume. Since theproduction of nitrogen oxides is a function of the transit time taken bycombustion gases to pass through the combustion chamber, high speedcombustion serves to reduce polluting emissions to a significant extent.

[0042]FIG. 3 shows an embodiment of a nozzle system. It comprises anozzle 24 that is fed with fuel. Primary and secondary air swirlers 26and 28 are disposed in such a manner as to feed the nozzle system withair in a radial direction. A Venturi 30 placed on the axis of the fuelnozzle 24 between the primary and secondary swirlers encourages the fuelto break up into a spray of fine droplets. Ventilation holes 32 whichopen out around and close to the tip of the nozzle 24 serve to limit oreven to eliminate any risk of coking on the tip.

[0043] The set of nozzle systems on the pilot head may also be providedwith fairing typically constituted by two caps 34 a and 34 b. Thisfairing serves to minimize head losses in the air that goes round thepilot head and serves to guarantee good air feed to the end wall of thetake-off head.

[0044] It should be observed that the combustion chamber may be made ofceramic matrix composite (CMC) material which, because of its ability towithstand high temperatures, makes it possible to achieve substantialsavings in terms of cooling air flow rate.

[0045] A combustion chamber with two heads that are offset and fittedwith nozzle systems in accordance with the invention can operate in thefollowing modes:

[0046] idling mode (or N/0) mode: fuel is injected solely via the pilothead fitted with its N nozzle systems of permeability PA. This mode isintended more particularly for lighting the engine and for operating itat speeds close to idling;

[0047] full throttle mode (FT or N/2N mode): fuel is injected over allof the nozzle systems with it being possible to modulate the way fuel isshared between the pilot and take-off heads. This mode is intended tocover most of the operating range of the chamber and it providesperformance that is better in terms of temperature, efficiency, andreducing polluting emissions; and

[0048] sector burning mode (SB or N/N mode): fuel is fed to all of thenozzle systems in the pilot head, and in general, to every other nozzlesystem in the take-off head. This mode of operation makes it easier toswitch between the pilot and take-off heads, particularly when thechamber end walls are of high permeability.

[0049] The disposition of the nozzle systems makes it possible to obtainan operating range for the combustion chamber that is significantlyextended, and to obtain lighting performance and stability that areequivalent to or better than a conventional chamber. Furthermore, thetransition between SB mode and FT mode can take place at low speed. Thetake-off nozzles all have the same individual permeability such thatswitching from SB mode (N/N) to FT mode (N/2N) is easier than in theengine of document FR 2 727 193 mentioned at the beginning of thisdescription where the n additional take-off nozzles present overallpermeability that is much greater than that of the n first nozzles.

[0050] The number of nozzle systems in the pilot head (i.e. N) isoptimized so as to reconcile lighting and stability performance andflame propagation while still allowing 2N nozzle systems to be installedin the take-off head. As an indication, the pilot head may be fittedwith 16 nozzle systems and the take-off head with 32 nozzle systems.

1/ An offset annular combustion chamber for an airplane engine gasturbine, the chamber comprising a pilot head having a plurality ofnozzle systems distributed on a pilot head chamber end wallinterconnecting an inner longitudinally-extending side wall of thechamber to a pilot head outer longitudinally-extending side wall, and atake-off head that is radially and axially offset from the pilot headand comprising a plurality of nozzle systems distributed on a take-offhead chamber end wall interconnecting the pilot head outerlongitudinally-extending side wall and a take-off head outerlongitudinally-extending side wall, the pilot head having at least Nsubstantially identical nozzle systems with overall permeability PAadapted to lighting and to speeds close to idling, and the take-off headhaving at least 2N substantially identical nozzle systems of overallpermeability PB, where PB is greater than or equal to PA, wherein thepermeability PA lies in the range 10% to 40% of the total air flow ratepenetrating into the chamber and the permeability PB lies in the range30% to 70% of the total air flow rate penetrating into the chamber. 2/ Achamber according to claim 1, wherein the permeability PA lies in therange 17% to 21% of the total air flow rate penetrating into thechamber. 3/ A chamber according to claim 1, wherein the permeability PBlies in the range 36% to 45% of the total air flow rate penetrating intothe chamber. 4/ A chamber according to claim 1, further comprising aplurality of external dilution orifices opening out at least through theouter longitudinally-extending side wall of the take-off head. 5/ Achamber according to claim 4, having at least one row of 2N outerdilution orifices opening out substantially perpendicularly to the outerlongitudinally-extending side wall of the take-off head. 6/ A chamberaccording to claim 4, wherein the air flow rate penetrating via theouter dilution orifices lies in the range 4% to 10% of the total airflow rate penetrating into the chamber. 7/ A chamber according to claim1, having a plurality of inner dilution orifices opening out through theinner longitudinally-extending side wall of the chamber. 8/ A chamberaccording to claim 7, wherein the air flow rate penetrating via theinner dilution orifices lies in the range 2% to 8% of the total air flowrate penetrating into the chamber. 9/ A chamber according to claim 1,wherein the nozzle systems of the pilot head and of the take-off headare disposed substantially in a staggered configuration. 10/ A chamberaccording to claim 1, wherein the axes of the nozzle systems of thepilot head and of the take-off head are directed towards a commonannular zone for exhausting the gases generated by combustion. 11/ Achamber according to claim 1, wherein the chamber end walls of the pilotand take-off heads are walls extending perpendicularly to the axis ofthe engine. 12/ A chamber according to claim 1, wherein the chamber endwalls of the pilot and take-off heads are walls of frustoconical shape.13/ A chamber according to claim 1, wherein each nozzle system of thepilot and take-off heads comprises a fuel nozzle, a primary air swirlerand a secondary air swirler that are fed radially, a Venturi situated onthe axis of the nozzle between the primary and secondary air swirlers inorder to encourage breaking up of the fuel into fine droplets, andventilation holes opening out close to a tip of the nozzle. 14/ Achamber according to claim 13, wherein the set of pilot head nozzlesystems is provided with fairing so as to minimize head losses in theair that flows round the pilot head.